In the wet end of a paper-making machine a forming screen or wire, supporting a slurry of cellulose fibers in water together with chemicals and pigments, slides over a number of dewatering elements which promote drainage of water from the slurry. Such dewatering elements include a forming board, foil blades, vacuum blades, suction box covers etc. The effluent water removed from the slurry through the forming screen typically contains about 0.5 to 1 percent of solid material. This solid material typically includes about 95 percent pigments (e.g. calcium carbonate) and about 5 percent cellulose fibers.
Hence, the forming screen sliding over these dewatering elements is subjected to extensive wear resulting from the sliding itself and from the presence of these pigments and cellulose fibers in the effluents. The forming screen, generally a polyester fabric, therefore has to be replaced for example every 30-35 days at a very high cost. Wear on the forming screen is particularly pronounced when the screen slides over the flat suction box covers, at which point the amount of effluent water has already been significantly reduced. Flat suction box covers are usually made of very hard ceramic materials, such as aluminum oxides, chromium oxides, zirconium oxides, silicon carbide or silicon nitride. The nature of such materials, including their surface roughness, porosity and pore size plays an important role in the wear of the forming screen, to a similar extent as the type and characteristics of the pigment in the water effluents (see for example M. Laufmann and H. U. Rapp, Wochenblatt für Papierfabrikation, 114/16, 615-622 (1986)).
The hard ceramic covers are vulnerable as they are subjected to accidental impact damage, stress cracking, thermal shock damage and sharpening under screen contact. Typically, their manufacturing costs are also very high as they consist of an assembly of small, 30 to 60 mm long individual elements which are glued together on the flat suction box, leaving small voids where pigment particles from the water effluent can accumulate. The retention of these pigment particles further accelerates the wear of the forming screen or wire.
Hence, there is a problem in the prior art relating to the wear on the forming screen at the wet end of paper-making machines due to the sliding of the screen over dewatering elements, and the associated high cost of the screen replacement. Moreover, there is a problem related to the vulnerability of the prior art ceramic materials.
GB 1 526 377 discloses dewatering elements having inserts made from polyurethane cast in situ and which are subsequently machined to the desired final shape. The preferred polyurethanes for use according to said patent are referred to-as having excellent hardness and abrasion properties, where the polyurethane has hardness values preferably in the range 93 Shore A to 96 Shore A. Minor amounts of fillers may be added to the polyurethane. As an example, the polyurethane “Adiprene L 167” is mentioned, which is a composition having a hardness of 95 Shore A. A small amount of green pigment is added to the composition.